involved in the Construction of Artillery. 149 



of the melting point of the zinc, it assumes very soon a crystalline structure, — 

 the crystals having their principal axes now all cutting perpendicularly through 

 the plate from side to side ; in other words, the j^lanes of internal structure being 

 in this and the former case absolutely turned round 180° of angular direction. 



15. The same change of structure takes place more strikingly in glass ; when 

 exposed for a considerable time to a heat short of fusion, or even of complete 

 softening, it is converted into tlie opaque substance known as Reaumur's porce- 

 lain, in which a crystalline structure is developed, and the principal axes are 

 arranged perpendicular to the surfaces recipient of the heat. 



Many other instances might be adduced, were this the place to pursue so 

 tempting a subject. But enougli has been given to indicate the generality of 

 the law. (NoteE.) 



4. — Molecidar Constitution of Cast-Iron. 



16. Now cast-iron is one of those crystallizing bodies which in consolidating 

 obeys, more or less perfectly, according to the conditions, this law also ; so that 

 generally it may be enunciated as a fact that in castings of iron the planes of 

 crystallization group themselves perpendicularly to the surfaces of external contour., 

 that is to say, in the directions in which the heat of the fluid cast-iron has passed 

 outwards from the body in cooling and solidifying. 



Because the crystals of cast-iron are always small, and are never very well 

 pronounced, these directions are seldom very apparent to the eye, but they are 

 not the less real. 



17. Their development depends: — 



1°. Upon the character of the cast-iron itself, whether it contain a large 

 quantity of chemically uncombined carbon (suspended graphite) 

 or not, which Karsten has shown to be the case with all cast-irons 

 that present a coarse, large-grained, sub-crystalline, dark, and 

 graphytic, or shining spangled, fracture ; such irons form in cast- 

 ings of equal size the largest crystals. 



2°. Upon the size or mass of the casting, the largest castings presenting, 

 for any given variety of cast-iron, the largest and coarsest aggre, 



VOL. XXIII. X 



